The automotive industry is more and more oriented in the design of engines, on-board systems and the vehicle itself, towards the reduction of fuel consumption, emission of pollutants, recyclability of materials and the recovery and conversion of thermal energy that would otherwise be dissipated.
As regards the recovery and conversion of thermal energy, the exhaust gas exiting from the internal combustion engine carries, after being treated by a catalyst or by an even more complex treatment unit, an amount of thermal energy that can be recovered for various useful purposes. For example, it has been suggested to recover the thermal energy from the exhaust gas for the purpose of heating the cabin (by more rapidly heating up the cooling water of the engine) of the vehicle, for the purpose more rapidly heating up the lubricating oil to the optimum temperature (lower viscosity) after a cold start of the engine, or for converting the thermal energy into electrical energy (with the aid of a suitable converter).
Such unit for recovering thermal energy is known, for example, from EP-A-2 381 083. The unit disclosed in this reference comprises an inlet for the exhaust gas, an outlet for the exhaust gas, and a heat exchanger arranged between the inlet and the outlet for the exhaust gas. In addition, the unit comprises a single rotatable valve flap that can be rotated to a recovery position in which the exhaust gas is directed along a U-shaped path through the heat exchanger while exhaust gas is prevented from flowing through the bypass. Alternatively, the valve flap can be rotated to a bypass position in which the exhaust gas is directed along a straight bypass of the heat exchanger, although the U-shaped flow path through the heat exchanger is not physically closed. As a further alternative, the valve flap can be rotated to any angular position between the recovery position and the bypass position for partial heat recovery.
However, the unit disclosed in EP-A-2 381 083 suffers from a number of disadvantages. First of all, the single valve flap may only substantially completely close the bypass even when the valve flap is in the recovery position, since the valve flap must be capable of being rotated to a desired position at any operating condition (e.g. after a cold start of the engine as well as when the engine is at the desired operating temperature, at which the inner diameter of the bypass may be larger than after a cold start due to the change in temperature). Accordingly, at least some clearance must be provided between the valve flap and the inner wall of the bypass so that the valve flap cannot completely close the bypass even when the valve flap is in the recovery position (bypass closed). As a consequence of this clearance, there is some backflow of exhaust gas that—after having flown through the heat exchanger—has been cooled to a temperature which is considerably lower than that of the exhaust gas coming from the engine or the catalyst and entering the unit. This leads to a loss of heat of the exhaust gas flowing through the heat exchanger which is unwanted and reduces the efficiency of the heat recovery process. Also, as the valve is in the recovery position the valve flap is rotated to a position in which it is arranged perpendicular to the direction flow of exhaust gas through the bypass. This may generate an unwanted backpressure which must be overcome thus reducing the efficiency of the engine.
As the valve is in the bypass position the U-shaped flow path through the heat exchanger is not completely closed, although any further heating up of the cooling water of the engine (which is the working fluid flowing through the heat exchanger) may be unwanted since the cooling water already has the desired temperature. Needless to say that in any position between the recovery position and the bypass position there may be backflow of exhaust gas which has been cooled—after having flown through the heat exchanger—to a temperature which is considerably lower than the temperature of the exhaust gas entering the system, thus substantially reducing the efficiency of the heat recovery process.